Chlorine dioxide in low concentrations (i.e. up to 1,000 ppm) has long been recognized as useful for the treatment of odors and microbes, see U.S. Pat. No. 6,238,643. Its use is particularly advantageous where microbes and/or organic odorants are sought to be controlled on and around foodstuffs, as chlorine dioxide functions without the formation of undesirable side products such as chloramines or chlorinated organic compounds that can be produced when elemental chlorine is utilized for the same or similar purposes.
Unfortunately, chlorine dioxide can be explosive at concentrations above about 0.1 atmosphere. Therefore, chlorine dioxide gas is not manufactured and shipped under pressure like other industrial gases, and conventional methods of on-site manufacture require not only expensive generation equipment but also high levels of operator skill to avoid generating dangerously high concentrations. These problems have substantially limited the use of chlorine dioxide to large commercial applications, such as pulp and paper bleaching, water treatment, and poultry processing, where the consumption of chlorine dioxide is sufficiently large to justify the capital and operating costs of expensive equipment and skilled operators for on-site manufacture.
Commercially, chlorine dioxide is produced from a variety of aqueous solutions of certain chlorine-containing salts, as disclosed for example in U.S. Pat. No. 5,009,875.
Attempts have also been made to produce chlorine dioxide using mixtures of solid reagents. Generally, the prior art has focused on three systems for chlorine dioxide production using solid reagents. One system employs a solid mixture of a metal chlorite and an acid in a liquid, aqueous environment. A second system combines a metal chlorite and a solid acid where chlorine dioxide gas is released under dry conditions. A third system employs the combination of a metal chlorite and a solid organic anhydride to generate a highly concentrated flow of chlorine dioxide which must be diluted with a constantly flowing stream of inert gas.
Aqueous solutions of chlorine dioxide are also known in the art. Two types of synthesis processes are generally used to provide chlorine dioxide solutions for commercial uses, such as poultry chiller water purification, wash-water purification, potable water treatment and as a teat dip for the control of non-human mammalian mastitis.
The first type of synthesis process is based on the manual combination of two aqueous solutions; one containing a source of chlorite anions and another being acidic. The solution containing chlorite anions is usually a solution of sodium chlorite having a concentration of between about 100 ppm and about 5% by weight and having a pH of about 13. The acidic solution may contain any acid capable of providing a pH below about 8.5 after the solutions are mixed. Such acids include citric acid, lactic acid, hydrochloric acid, sulfuric acid, and dissolved carbon dioxide (i.e., sodium bicarbonate). The antimicrobial performance of the resultant solutions depends upon the degree to which the chlorite anions from the chlorite source solution are converted to free molecular chlorine dioxide (“Chlorine Dioxide”) in the solution, as chlorine dioxide is the effective agent for both antimicrobial and deodorization activity.
In one variation on this synthesis process, the pH of the sodium chlorite solution is reduced from about 13 to about 8 using the acidic solution. Chlorite anion is thus converted to chlorine dioxide via the reaction below.5ClO2−+5H+4ClO2+HCl+2H2OSuch solutions having a pH of about 8 are generally referred to in the industry as “stabilized” chlorine dioxide solutions, and usually contain between about 100 ppm and 5% of a mixed solution of chlorine dioxide and unconverted chlorite anion. Because the acid concentration is relatively low at a pH of 8, the typical ratio of chlorine dioxide to chlorite anion in a stabilized chlorine dioxide solution is less than 0.01. Therefore, for a given initial concentration of chlorite anion, stabilized chlorine dioxide solutions are relatively weak antimicrobial agents due to their low conversion of chlorite anion to chlorine dioxide. Also, since the stabilized chlorine dioxide solutions are typically supplied at a concentration of less than about 5% by weight sodium chlorite, shipping and storage of the solution is relatively expensive due to the high weight of water that must be transported with the sodium chlorite.
Chlorite anion is generally stable in stabilized solutions (pH 8) and thus has an advantageously long shelf life. However, the chlorite anions are typically activated just prior to use to improve effectiveness. This is accomplished by the addition of a strong acid to lower the pH to below about 3.5 and convert more chlorite anion to chlorine dioxide via the reaction shown above. Since the activation process involves the addition of a strong acid to lower the pH, a high level of operator skill is required to handle, measure and mix the acid with the stabilized chlorine dioxide solution. Also, since the activation process results in a solution having a pH of less than about 3.5, such activated solutions are not well suited to work in combination with, for example, detergents which work best under alkaline or neutral pH conditions. Contact of these solutions with many metals should also be limited because of possible metallic corrosion by the acidic solution.
Such activated solutions typically have a ratio of chlorine dioxide to chlorite anion below about 0.05 when the solution is acidified to a pH of about 3. A higher ratio of chlorine dioxide to chlorite anion can be achieved in such activated solutions, but doing so is dangerous and requires extreme operator skill. Achieving a ratio of chlorine dioxide to chlorite anion above about 0.05 requires further acidification to a much lower pH than 3 (typically less than 2) and often requires that the further acidification be performed at concentrations of chlorite anion above about 5000 ppm. Under such conditions of extremely low pH and high chlorite ion concentration a sufficient chlorine dioxide concentration can be generated in such solutions such that the vapor pressure of gaseous chlorine dioxide in equilibrium with the solution approaches the explosive range. Therefore, manual acidification (i.e. without chlorine dioxide generation equipment as discussed below) is not commonly employed in producing solutions having a high ratio of chlorine dioxide to chlorite anion.
In the second type of chlorine dioxide solution synthesis process, chlorine dioxide solution is generated from either a sodium chlorite solution or stabilized chlorine dioxide solution using chlorine dioxide generation equipment at the point of use. The generated solution typically has a ratio of chlorine dioxide to chlorite anion of between about 10 and 25, and as a result such solutions are highly effective antimicrobial agents. Since generated chlorine dioxide solution is typically used shortly after generation, the relatively high decomposition rate of chlorine dioxide in solution is unimportant. Also, since aqueous sodium chlorite is commercially available at higher concentrations than are typically available in the form of stabilized chlorine dioxide solutions, the cost of storing and shipping the aqueous sodium chlorite solutions can be lower when compared to stabilized chlorine dioxide solution. However, the high cost of the chlorine dioxide generation equipment and the high level of operator skill needed for its operation make generated chlorine dioxide solution best suited to relatively large applications such as water treatment and poultry processing where the consumption of chlorine dioxide is sufficiently large thereby justifying the such high capital and operating costs.
In addition to the two types of commercial synthesis processes for chlorine dioxide solution discussed above, solutions containing chlorine dioxide and having a high ratio of chlorine dioxide to chlorite anion can be generated by absorption of gaseous chlorine dioxide into water. Chlorine dioxide is first produced in solution by conventional means, e.g. acid activation of a solution of sodium chlorite. Inert carrier gas, typically air or nitrogen is then bubbled through the activated solution picking up some of the chlorine dioxide. That gaseous mixture of chlorine dioxide and carrier gas is then bubbled through a second vessel containing water where the chlorine dioxide is dissolved to produce a solution of chlorine dioxide typically having a ratio of chlorine dioxide to chlorite anion of about 20 or higher. While it is possible to produce substantially pure solutions of chlorine dioxide in this manner, it requires a very high level of operator skill and is rarely done outside of the laboratory.
Attempts have been made to reduce the cost of generating chlorine dioxide solutions by using mixtures of alkaline chlorite salts and acidic dry powders which, upon addition to water, acidify the water and generate chlorine dioxide via reaction described above. U.S. Pat. No. 2,022,262, discloses stable stain removing compositions comprising a dry mixture of a water soluble alkaline chlorite salt, an oxalate, and an acid. Since alkaline chlorites are strong oxidizers and corrosively caustic, a relatively high level of user skill is needed to employ this process.
U.S. Pat. No. 2,071,094 discloses deodorizing compositions in the form of dry briquettes comprising a dry mixture of a soluble chlorite, an acidifying agent, and a filler of lower solubility. Generation of chlorine dioxide begins as the briquette dissolves in water. This process is suitable for unskilled users, but still requires that the resultant solution be produced at an acidic pH.
U.S. Pat. No. 2,482,891 discloses stable, solid, substantially anhydrous compositions comprising alkaline chlorite salts and organic acid anhydrides which release chlorine dioxide when contacted with water. The patent disclosure indicates that the preferred solution is highly concentrated and consequently would have been at an acidic pH. As such, this process suffers from the same limitations as the '262 and '091 patents mentioned above.
U.S. Pat. No. 4,585,482 discloses a long-acting biocidal composition comprising a chlorine dioxide liberating compound and a hydrolyzable organic acid-generating polymer. Methods are disclosed for producing dry polymer encapsulated microcapsules containing such compositions and water such that the resultant dry materials release chlorine dioxide gas. The primary purpose of the polymer encapsulating film of the '482 patent is to provide for hard, free flowing particles, and to protect against the loss of water from the interior of the microcapsule. Immersing the microcapsules in water would produce a chlorine dioxide solution. Producing chlorine dioxide solution in this manner would eliminate the complications of measuring and mixing reagents and the cost of capital equipment that characterize the prior art. In addition, the solution pH need not be acidic so it would be feasible to produce chlorine dioxide in a detergent solution. However, the materials of the '482 patent are not storage stable because chlorine dioxide is released soon after manufacturing. Furthermore, chlorine dioxide is released over a period of several days, so the materials of the '482 patent are unsuitable for quickly preparing a useable chlorine dioxide solution. Finally, once mixed in water the microcapsules cannot be removed from the water in a simple fashion. Typically the microcapsules must be separated by a process such as filtration.
The present assignee manufactures Aspetrol® chlorine dioxide generating tablets. The tablets are used in a wide array of applications such as to oxidize foul smelling compounds, deodorize areas, disinfect, etc. Assignee's patents directed to chlorine dioxide generating tablets include U.S. Pat. Nos. 6,699,404 and 6,432,322. These patents disclose solid bodies for preparing highly converted solutions of chlorine dioxide when added to water. The solid body comprises a metal chlorite such as sodium chlorite, an acid source such as sodium bisulfate and, optionally, a source of free halogen such as the sodium salt of dichloroisocyanuric acid or a hydrate thereof.
U.S. Pat. No. 6,238,643, also issued to the present assignee, discloses a method of producing an aqueous solution of chlorine dioxide from the reaction of chlorine dioxide generating components. The chlorine dioxide generating components are a metal chlorite and an acid forming component which do not react to produce chlorine dioxide in the substantial absence of water. The chlorine dioxide generating components are disposed in an enclosed space bounded at least in part by a membrane that is water and/or water vapor permeable but impermeable to the chlorine dioxide generating components contained therein. The membrane-bounded space containing the chlorine dioxide generating components is contacted with liquid water so the chlorine dioxide may generate and pass out through the membrane into the liquid water forming the aqueous solution of chlorine dioxide.
One problem with chlorine dioxide compositions, in particular, those formed with alkali metal chlorites and acid, whether solid or liquid, is that the resultant composition results in an acidic solution that is corrosive to metals. Thus, a chlorine dioxide solution is desired that will sanitize and cleanse especially metal objects without harming substrates on which it is being used. In accordance with the present invention, a chlorine dioxide solution is provided that inhibits metal corrosion.